1. Field of Invention
The present invention relates to high efficiency electronic ballast for a large power (250 W, 400 W) metal halide lamp. More specifically, the invention related to a high efficiency electronic ballast for a metal halide lamp that operates at 100 k-220 kHz (for 400 W lamp) or 180 k-300 kHz (for 250 W lamp) with different modulation at different operation period Consequence, the electronic ballast can operate the large power (250 W, 400 W) metal halide lamp very stable from starting to normal operation without the disadvantage of the complicated of low frequency with the high loss and low efficiency compare with the higher frequency operation. The efficiency of the electronic ballast is typically greater than 95%. The invention related to high efficiency electronic ballast for large power (250 W, 400 W) metal halide lamp that has two DC block output capacitor to isolate the ground and lamp leads. Therefore, the ballast is well protected when the lamp leads shorted to ground and without an isolation transformer which has low efficiency.
2. Description of Related Arts
There U.S. Pat. Nos. 5,796,216, 6,181,076, 6,936,699 disclose the ballast with an isolation output transformer. The ballasts are well protected when the output (lamp) leads are shorted to ground. The drawback is that the isolation transformer has very high transfer loss and generates a lot of heat. Therefore, the efficiency of the ballast is lower and reliability of the ballast is lower as well.
U.S. Pat. No. 6,768,274 discloses a lamp to ground fault protection circuit. This circuit checks the balance of the in and out lamp current to determine whether it is short to ground or not. During the check period, the transistor has to pass a large surge current. It will cause damage to the transistor.
U.S. Pat. No. 5,942,859 discloses the ballast with a lower frequency square wave output circuit. The circuit is quite complicated. The cost is high. The efficiency is lower.
U.S. Pat. No. 6,426,597 discloses the ballast with an improved lower frequency square wave output circuit. The overall circuit is still quite complicated. The cost is high. The efficiency is improved a little.
Some ballasts operate around 20 k-50 kHz with modulation of sine wave, triangle wave, square wave, white noise, or random code. It works with some kinds of lamps without the acoustic resonance. However, it will not work for all kind of lamps.
Other ballasts operate very high frequency which is above the acoustic resonance frequency, such as 400 kHz for 400 W and 500 kHz for 250 W. Due to the high operation frequency, the transistor loss is increased. The EMI noise is increased as well. Therefore, the ballast cost is high and the ballast lose is high as well.
With deep frequency, modulation will cause the metal halide lamp flicker in starting phase. U.S. Pat. No. 6,696,800 discloses a method to avoid flicker when MH lamp starts using deep frequency modulation. It detects MH lamp voltage to determine the modulation deep rate, it also can avoid the starting phase flicker of the MH lamp, but the drawback is the detect lamp voltage circuit cost more and is complexity.